1. Field of the Invention
The present invention relates to a semiconductor optical device such as a semiconductor laser device used as a light source for optical information processing, a signal for optical communication, an excitation light source of a fiber amplifier, or the like, a semiconductor amplifier for amplifying an optical signal, or an optical modulator for modulating an optical signal.
2. Description of the Background Art
An end face, including a waveguide layer, of a semiconductor laser device or a semiconductor optical device, such as an optical modulator, is generally coated with a reflecting film. When a reflecting film having a refractive index n1 on the end face portion of the semiconductor element has a film thickness d equal to an odd integer multiple of λ/(4n1), the reflectance of the reflecting film becomes a minimum value. In addition, when a coating having a refractive index which is a square root of a refractive index nc of a laminated element is on a waveguide layer at the end face portion is, an antireflecting film is obtained. For example, the reference of I. Ladany, et al., “Scandium oxide antireflection coatings for superluminescent LEDs”, Appl. Opt. Vol. 25, No. 4, pp. 472-473, (1986), describes a semiconductor laser with an antireflection film on the end face.
Wavelength dependence of reflectance of a reflecting film (refractive index n1=1.449) including films of various thicknesses in a laminated element (effective refractive index nc=3.37), including a waveguide layer of an end face portion of a semiconductor optical device, will be considered. In this case, the reflectance is set to be the minimum value at a setting wavelength λ=980 nm. When the reflectance is the minimum value, the thickness is an odd integer multiple of λ/(4n1). If the single-layer reflecting film has a thickness of λ/(4ni) and if the single-layer reflecting film has a thickness of 5λ/(4n1), it is understood that a flat portion near a minimal value of the reflectance in the single-layer reflecting film having the thickness of λ/(4n1) is larger than that in the single-layer reflecting film having the thickness of 5λ/(4n1).
When a thickness d of the reflecting film on the end face portion of the semiconductor optical device is increased an odd-number of times λ/(4n1), a wavelength band of a low-reflectance area near the minimal value of the reflectance becomes narrow, and a semiconductor laser characteristic disadvantageously varies singificantly due to the wavelength dependence of the reflectance of the reflecting film.
Typically, the single-layer reflecting film having a thickness of d1=λ/(4n1) has a minimal reflectance of 4% at a wavelength λ of 980 nm. In this case, the reflectance in the range of a wavelength of 848 nm to a wavelength of 1161 nm ranges from a minimal value of 4.0% to 6.0%. The continuous wavelength band in the range of 4.0% to 6.0% is 313 nm. Meanwhile, the single-layer reflecting film having a thickness of d1=5λ/(4n1) has a minimal reflectance of 4% at a wavelength λ of 980 nm. In this case, the reflectance in the range of a wavelength of 951 nm to a wavelength of 1011 nm ranges from a minimal value of 4.0% to 6.0%. The first continuous wavelength band in the range of 4.0% to 6.0% is 60 nm narrower than that of the single-layer reflecting film having a thickness of d1=λ/(4n1). Then, a first reference value is obtained by dividing the wavelength band by the predetermined wavelength of 980 nm is about 0.061. Also, the reflectance in the range of a wavelength of 949 nm to a wavelength of 1013 nm ranges from a minimal value of 4.0% to 6.5%. The first continuous wavelength band in the range of 4.0% to 6.5% is 64 nm. Then, a second reference value is obtained by dividing the wavelength band by the predetermined wavelength of 980 nm is about 0.065.